Conditions for maintenance of embryonic chick duodenum in organ culture in serum-free medium, responsive to vitamin D and several analogues, with excellent preservation of mucosal structure, have been essentially optimized. Utilization of this unique system has provided valuable insights into the intestinal calcium absorptive mechanism. It is proposed to further investigate the regulation of synthesis and physiological role(s) of CaBP under the precisely controllable in vitro conditions made possible only through the use of this system. Experiments to date using this system have established that vitamin D itself induces de novo CaBP synthesis and stimulates duodenal calcium absorption by a calcium-dependent mechanism. To further elucidate this mechanism, attempts will be made to assess the sub-cellular localization of vitamin D, to isolate and characterize hypothetical vitamin D-receptors and CaBP-mRNA, and to assess the role of calcium (and, perhpas, phosphate) on CaBP induction. There will be considerable further investigation of the interaction between vitamin D, CaBP and the adenyl cyclase system, originally discovered using this system. The possible involvement of bone factors in the regulation of intestinal calcium absorption will be explored further and attempts made to isolate such factors. Extension of currently successful reconstitution experiments will be undertaken with emphasis on CaBP localization and measurement of transport kinetacs. The vitamin D-mediated mechanism(s) of absorption of nutrients other than calcium, eg., Co, Fe and PO4, etc., will be investigated. Although evidence obtained thus far indicates no effect of PTH or CT on the calcium-absorptive mechanism of the cultured intestine, further work with these and other agents, EHDP, methylcholanthrene, cortisone, phenobarbitol, etc., will be done. Finally, the possible use of the extreme sensitivity of the organ cultured intestine to 1,25-(OH)2-D3, in terms of CaBP induction, as a bioassay in animal and human blood will be studied with a view toward assessing the role of this sterol in health and disese.